Acid rain has long been recognized as a threat to lakes, forests, and other regions, both natural and human made. The list of ways in which it is known to pose a threat continues to grow longer.
Recent research has added to that list the ability of acid rain to weaken or destroy forests by leaching essential metal nutrients such as potassium, calcium, and magnesium from the topsoil. A team of researchers headed by University of California Riverside Earth Scientist Martin Kennedy has shown that the presence of these elements in the topsoil is more important than previously believed.
If deprived of a certain critical nutrient, such as calcium, a tree faces the risk of dying. In parts of Germany, for example, trees are already dying not from the direct effects of the acid, but from magnesium deficiency stemming from leaching by acid rain.
Kennedy explained that "In our study, we were attempting to determine what fraction of the total elements available in the soil the plants could access. We found it was a very small proportion."
It has long been thought that trees obtain their essential metal nutrients from weathered rock particles deep in the soil. But by demonstrating that the trees obtain these nutrients almost exclusively from atmospheric sources, Kennedy and colleagues suggest that the trees cycle a small pool of nutrients that are continually replaced by dilute atmospheric sources. The scientists experimented on trees in the unpolluted forests of southern Chile.
"We went to the cleanest atmosphere on earth, "Kennedy says. "There, we sampled soils, stream water, rain and plants, and analyzed the strontium isotope composition."
Strontium isotopes indicate very accurately which fraction came from the rain and which fraction came from the rock. "We found that in the dominant tree species - the southern Beech - approximately 90% of the strontium, and thus other similar nutrient elements, were brought in by the rain and did not come from soils or rocks, as just about everyone had assumed," said Kennedy.
The researchers also applied a distinctive artificial chemical tracer to the soils in a small portion of the Chilean forest. The tracer mimics the natural nutrients in the soils and trees with the advantage that it can be measured and observed as it moves throughout the soil plant system. By sampling the trees and soil over time and by analyzing the samples for the tracer, the scientists found that within three years most of the tracer was quickly leached from the topsoil. The loss of this element within such a short amount of time surprised the researchers because it implies that a far smaller pool of nutrients is available to the trees from the upper soil than they had imagined.
"The small size of this upper soil nutrient pool has important implications for industrially influenced forests in the northeastern United States and in Europe," said Kennedy. "These forests may be more vulnerable to the effects of acid rain than we had previously thought."
Hydrogen ions from the acid in acid rain replace the nutrient elements in the soil. For every unit of acid added to the soil, an equivalent amount of nutrient elements is removed. As a result, more nutrients get leached from the soil than arrive from weathering of rocks or precipitation.
"Our study not only challenges the dominant paradigm that rocks and soil mineral weathering provide a majority of some important plant nutrients like calcium and potassium," said Kennedy, "but it also proposes that our 'stable' old growth forests are the most at risk from acid rain, and that it is a bigger problem, potentially, than we ever imagined."